Computer digital data storage devices are required to provide increasingly high data storage capacities and data transfer rates. This is particularly true with the advent of the so-called "world wide web" which is consuming ever-increasing amounts of digital content information. One family of storage devices suitable for storing vast amounts of information content is streaming tape. Streaming tape drives are so named because multiple blocks of user data are typically written to, or read from, tape in a single streaming operation, rather than as a series of start-stop operations. Streaming tape is particularly well suited for backup operations as well as for providing archival and retrieval operations for vast quantities of information content. In this regard, optical storage is also known for voluminous content storage and retrieval.
Tape libraries are known in the art. One example of a tape library is provided by the Ostwald U.S. Pat. No. 5,236,296. In this patent, a tape library is described in FIG. 8 as comprising a vast, semi-cylindrical array of tape cartridge storage slots aligned generally along a fixed radius of curvature. A central cartridge inventory is maintained by a library controller, so that logical requests for a particular drive and cartridge may be translated by the library controller into physical device locations and electromechanical operations. In this prior example, a robotic arm rotating at a focus of the cylindrical segment is elevated and rotated to a particular cartridge storage slot. A picker-gripper mechanism of the arm then "picks" and "grips" the cartridge stored in the slot and moves the cartridge out of the slot and into a temporary transport slot of the arm. The robotic arm is then commanded to perform a second rotation/elevation operation in order to present the retrieved tape cartridge to a loading tray of the selected tape drive, and the drive then loads the cartridge and threads the tape for recording/playback operations, following initial setup and calibration routines conventional with tape drives. The drive may be one of several drives accessible by the robotic arm.
One obvious drawback of this prior "move media cartridge to drive" approach is the time delay incurred between receiving the tape cartridge request and having the cartridge ready for user data operations at the selected tape drive. Another drawback of this prior approach is that a single robotic arm mechanism services hundreds, if not thousands, of tape cartridge slot locations, and any failure of the arm mechanism essentially removes all of the tape cartridges it services from automated access within the library. This prior approach permits scaling only in the sense of selectively filling up the available tape cartridge slots with tape cartridges.
Some improvement over the above approach is described in Dimitri et al. U.S. Pat. No. 5,377,121 for "Automated Storage Library Having Inventory at Picker Level". This patent describes an optical disk library of four storage boxes with a single personal computer acting as a library controller for two optical disk library storage boxes. Each storage box includes a picker controller which maintains a physical inventory of optical disks accessible by a controlled disk picker gripper mechanism within the particular box. In response to requests from the library controller, the picker controller decides where an optical disk is located and whether it must be moved, and issues appropriate commands to the mechanism to move the requested disk to an optical disk drive also present within the particular storage box. A discrete switching structure comprising separate switches is used to route information between the library controller and each selected optical disk drive.
In the preferred example described in the Dimitri et al. patent, two optical disk lists are included in the internal data structures of the two described library controllers. Each optical disk list includes the label of each optical disk for which the particular library controller is responsible, and the particular storage box containing the desired disk, but not having any information as to the actual physical location of the disk. Each picker controller maintains its disk location inventory list in a non-volatile, battery backed-up random access memory. Each disk inventory maintained by the picker includes disk identification/location information only for the disks present within the particular storage box serviced by the picker controller. As optical disks are inserted into, withdrawn from, or moved about a particular storage box of the library, the respective picker controller updates its optical disk inventory. While this approach represents an improvement over the tape library approach of the Ostwald patent, the approach essentially follows a "media-to-drive" approach with the latencies and drawbacks already noted for such approach.
It is also known in the art to physically move the media drive to a particular media cartridge location and thereupon load a cartridge at the location. Kvifte et al. U.S. Pat. No. 5,337,297 entitled: "Magazine Drawer Manipulation for a Data Carrier Loader" shows one approach. While this approach works, it has the same latencies expected of the media-to-drive approach, in that a massive object, the drive, must be translated to a cartridge location, and this movement operation requires a certain time interval.
Tape cartridge loaders provide a relatively low cost solution for very small data library applications. Typically, a cartridge loader includes a removable cartridge magazine that has slots for media cartridges, such as tape cartridges. A pass-through picker-gripper mechanism is positioned within the loader by a servo mechanism, and passes a selected cartridge into a transport tray, and then relocates to an oppositely facing media drive whereupon the media cartridge is loaded automatically into the media drive. The Schneider et al. U.S. Pat. No. 5,231,552 which is assigned to the same assignee as the present application describes a magazine and receiver for a cartridge loader. The loader mechanism is shown in mechanical overview in FIGS. 16A, 16B and 17 in the '552 patent. Media cartridge loaders have been improved and refined to support additional magazines and media drives, and media cartridge pass-through arrangements have been proposed for passing a selected cartridge from a first loader to an adjacently mounted second loader. While an effective, low cost, low capacity data library may be provided by using a media loader, a continuing problem remains because the media loaders also follow the conventional, traditional media-to-drive approach, discussed above.
A further approach for increasing the number of media cartridges which may be handled within a library environment is what is known as a "pass-through" arrangement. Commonly assigned, copending U.S. patent application Ser. No. 08/710,033 filed on Sep. 11, 1996, for "A Multi-Drive, Multi-Magazine Mass Storage and Retrieval Unit for Tape Cartridges" presents one example of a cartridge pass-through architecture.
Typically, media loaders operate in accordance with a standardized command structure. One such command structure is found in the Small Computer System Interface-2 draft standard X3T9.2 Project 375D (ANSI X3.131-199X). In this particular industry specification, a medium changer device includes a medium transport element, at least one storage element, and a data transfer element. An import/export element may also be supported. A storage element is identified as a storage slot for storing a standard medium unit, such as a disk or a tape cartridge. Typically, in order to access data on a standard medium unit, a host system will have to issue commands to both the medium loader and to the drive. The commands to the loader may include "move medium"; or, "exchange medium" and "read element status". Commands directed by the host to the drive may include "test unit ready", "inquiry", "start-stop" and "load-unload" commands, in addition to the obvious "read/write" commands. One important characteristic worth noting about this command structure is that the logical address of the drive is supplied to the media loader as a destination, as well as to the drive itself for subsequent read or write operations from or to the selected and automatically loaded medium unit.
A hitherto unsolved need has remained for a more effective architecture for an expandable digital storage media library having vectored drive addressing for overcoming significant limitations and drawbacks associated with the prior approaches.